The present invention relates to electrical cable assemblies and, more particularly, to flex circuits connected to a series of connectors in which intercircuit cross-talk is minimized and in which impedance changes within each circuit path are likewise minimized.
In electronic systems that use printed circuit boards, one of the trends is to provide a series of printed circuit boards that communicate with one another along a multi-circuit bus via edge-mounted headers. In one type of design, the printed circuit boards are interconnected via a backplane or motherboard which carries connectors that receive the edge-mounted headers of each printed circuit board and effects interconnection via printed conductor traces on the motherboard. In other systems, cable assemblies are used to interconnect the printed circuit boards. These cable assemblies include multi-contact connectors at the opposite ends of the cable as well as one or more similar connectors positioned along the cable at intermediate positions. In general, each circuit path in the cable connects like contacts in each of the connectors.
While discrete wire bundles and flat-conductor cables have been used in the cable assemblies, one trend is to use flex circuits to interconnect the various contacts in the connectors. In one form, the flex circuit typically includes a flexible polyimide substrate having a plurality of printed conductive traces on one side with a polyimide insulating cover providing a complete assembly. In this flex circuit arrangement, every other conductive trace carries a signal with the alternate conductive traces connected to ground so that every signal path is bounded on opposite sides by a ground path to minimize cross-talk. The spacing between the signal carrying path and its adjacent ground conductors is critical and small dimensional changes can adversely affect cross-talk. In another type of flex circuit, the flexible polyimide substrate includes a plurality of printed conductive traces on one side and a continuous conductive ground plane on the other side with polyimide insulating covers on opposite sides of the conductive trace, substrate, and ground plane laminae to provide a complete cable structure. When the spacing between the conductive traces on the one side of the substrate and the ground plane is less than the spacing between the conductive traces on the other side of the substrate, cross-talk is a function only of the spacing between the ground plane and the conductive traces, this spacing being only the thickness of the substrate upon which the conductive traces and the ground plane are mounted.
The latter flex circuit configuration offers increased signal carrying capacity over the former configuration, since each conductive trace carries a signal. In general, electrical connection between the conductive traces and the connectors for the latter flex circuit configuration is effected by removing a portion of the insulating cover and the ground plane and forming a hole in the substrate and conductor path through which hole the tail portion of the respective contact in the connector housing is passed to effect an electrical connection. This connection arrangement is generally satisfactory, although any penetration of the ground plane or the conductive trace establishes a two-dimensional discontinuity that alters the characteristics of the connection path and the shielding effect offered by the ground plane. Thus, the closely adjacent conductive traces in the flex circuit can provide different impedances, including resistive, capacitive, and inductive constituents, for the various signal-carrying circuits as a function of the two-dimensional discontinuities caused by the various holes. In addition, the presence of a hole in the conductor trace requires an enlargement of the conductive trace in the vicinity of the hole to provide an adequate solderable surface for connection to the tail portion of the respective contact. While this type of conductive trace arrangement does not cause signal transmission degradation at DC and at relatively low-frequencies, the presence of the discontinuities in either or both the signal carrying conductive traces and the ground plane can cause signal degradation at relatively high digital speeds, especially where the interconnects are part of a digital bus application in which all pulses must travel in a synchronous or quasi-synchronous manner with minimal relative degradation because of the impedance characteristics of the flexible circuit.